header:before{position:absolute;bottom:0;content:"";width:100%;height:1px;background:linear-gradient(90deg,hsla(0,0%,100%,0) 0,hsla(0,0%,100%,.3) 50%,hsla(0,0%,100%,0));transition:opacity .45s cubic-bezier(.6,.6,0,1);z-index:1}